Gastric stimulator apparatus and method for use

ABSTRACT

Apparatus and method for stimulating neuromuscular tissue in the stomach. The neuromuscular stimulator stimulates the neuromuscular tissue by applying current-controlled electrical pulses. A voltage sensor detects the voltage across the neuromuscular tissue to determine if the voltage meets a predetermined voltage threshold. A control circuit adjusts the current-controlled pulse if the voltage is found to meet the voltage threshold, such that the voltage does not exceed the voltage threshold. A voltage-controlled pulse may also be applied to the tissue. A current sensor would then detect whether the current on the neuromuscular tissue meets a predetermined current threshold, and a control circuit adjusts the voltage-controlled pulse such that the current does not exceed the current threshold. A real time clock may be provided which supplies data corresponding to the time of day during the treatment period. A programmable calendar stores parameters of the stimulating pulse, wherein the parameters have a reference to the time of day.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.09/466,387, filed on Dec. 17, 1999, now U.S. Pat. No. 6,895,278 whichclaims the benefit of U.S. Provisional Application No. 60/129,209, filedApr. 14, 1999 and is incorporated by reference in its entirety herein.

BACKGROUND OF THE INVENTION

This invention relates to electrical stimulation apparatus and methodsfor use in stimulating body organs, and more particularly to implantableapparatus and methods for periodic electrical gastric stimulation.

The field of electrical tissue stimulation has recently been expanded toinclude devices which electrically stimulate the stomach with electrodesimplanted in the tissue. These gastric stimulators have been found tosuccessfully combat obesity in certain studies. Medical understanding asto how this treatment functions to reduce obesity is currentlyincomplete. However, patients successfully treated report achievingnormal cycles of hunger and satiation.

An apparatus and treatment method for implementing this therapy wasdescribed in U.S. Pat. No. 5,423,872 to Dr. Valerio Cigaina, which ishereby incorporated by reference in its entirety herein. The apparatusdescribed in the Cigaina patent stimulates the stomach antrum pyloricumwith trains of stimulating pulses during an interval of about twoseconds followed by an “off” interval of about three seconds.

Current pacemaker design incorporates a number of features useful forthe type of tissue or organ being stimulated. Pacemakers stimulatingcardiac or neurological tissue, for example, may typically contain anaccurate, drift-free crystal oscillator to carry out real-time functionssuch as pulse generation. In particular, some cardiac pacemakers use atime reference to keep track of the time-of-day with a 24-hour clock inorder to log data or to vary pacing parameters during the 24-hour cycle.Similarly, neurological stimulators, such as a neurological stimulatormanufactured by Cyberonics, may use the time-of-day as a reference todeliver one or more periods of pulse-train stimulation (typicallylasting a few minutes each) to the vagus nerve to treat epilepsy.

The design and operation constraints for a gastric pacemaker, orstimulator, are substantially different from those for a cardiacpacemaker or a neurological pacemaker, for example. With a gastricstimulator for weight loss, size is less of a concern because of thelarge anatomy associated with obesity. However, a long operating lifefor an implantable device remains an important feature, given thesignificantly higher current drain required by this therapy compared tocardiac pacing. Since the implantable pulse generator may be locatedsubcutaneously in the abdominal wall, it is feasible to use a largerdevice, including a larger, longer-life battery.

Moreover, stomach stimulation may require different levels and cycles ofstimulation than that required for cardiac stimulation or nervestimulation. In a neuromuscular stomach generator, for example, powerconsumption can be five to seven times higher than for a cardiacpacemaker. Maintaining the proper energy level for stimulation may placeenergy demands on the life of the battery. The characteristics ofentrainment of the stomach tissue may require cycling of the electricalstimulation in more complex schedules than that previously required.Observations of early human implants have shown a surprising increase inthe impedance of the electrode tissue interface, from about 700 ohms attime of implant to 1300 ohms after only as much as three months ofimplants. With constant current and increased impedance, voltage drainon the battery may be unacceptably high.

Thus, there is a need to optimize the operation of gastric pacemakers,or stimulators, so as to provide a longer life for the device, andhence, a longer duration of therapy without the need for repeatedsurgical procedures.

It is an advantage to provide an apparatus and method of stimulationwherein voltage or current can be controlled to extend the useful lifeof a battery used therein.

It is also an advantage of the invention to provide an apparatus andmethod of stimulation that is able to calculate and store dataparameters to improve the levels of stimulation based on operatingconditions.

It is a further advantage of the invention to provide a clock functionwhich allows the stimulation cycles of the tissue to be programmed andexecuted on long term basis.

SUMMARY OF THE INVENTION

These and other advantages of the invention are accomplished byproviding apparatus and methods for stimulating neuromuscular tissue ofthe gastrointestinal tract by applying an electrical pulse to theneuromuscular tissue. The electrical pulse applied to the tissue may bea current-controlled pulse or a voltage-controlled pulse as deemedappropriate by one skilled in the art. In the case of a stimulatorapplying a current-controlled pulse, the stimulator may include avoltage sensor to sense the voltage across the neuromuscular tissuebeing stimulated. A voltage threshold is determined by the circuitry. Ina preferred embodiment, the voltage threshold may be adjustable and maybe a function of the level of current applied to the tissue beingstimulated.

The circuitry compares the sensed voltage and the predetermined voltagethreshold. If the sensed voltage is found to meet or to exceed thepredetermined voltage threshold, the circuitry will adjust thecurrent-controlled pulse such that the sensed voltage does not exceedthe predetermined voltage threshold. In a preferred embodiment, this maybe accomplished by generating an error signal between the sensed voltageand the voltage threshold by using negative feedback control. Theoccurrence of the sensed voltage meeting or exceeding the predeterminedvoltage threshold may be stored as an “event”, along with time at whichthe event occurred during the pulse interval and/or during the treatmentperiod.

The circuitry also provides the capability of utilizing the data that isobtained during the sensing and feedback functions. For example, thetotal impedance may be calculated from the voltage and current values.One component of the impedance may be the electrode resistance, and thesecond component may be the polarization capacitance. The electroderesistance may be obtained by dividing the voltage by the controlledcurrent. The capacitance may be obtained from the current divided by thetime rate of change of the voltage. The calculated values of theresistance and the capacitance may be stored on a memory device ordisplayed on a display device, or used in the feedback process todetermine the increment of adjustment to the current-controlled pulse.

The neuromuscular stimulator may also include a real time clock and aprogrammable calendar for tailoring the stimulating waveform parametersover the treatment period. The real time clock supplies datacorresponding to the time of day during the treatment period. Theprogrammable calendar stores parameters which refer to the shape of thestimulating waveform. Each of the parameters may be referenced directlyor indirectly to the time of day. Circuitry, such as a control circuit,applies the stimulating pulses which are defined by the parameters atthe appropriate times of the day during the treatment period.

In a preferred embodiment, the parameter may be a time period duringwhich the electrical pulses are applied. The time period may be definedby a start time and a duration. When the time period is so defined, thecircuit may apply the stimulating pulse beginning at the start time andcontinuing for the specified duration. The time period may alternativelybe defined by a start time and a stop time. In such a case, the circuitapplies the stimulating pulse beginning at the start time, and continuesto apply the pulses until the stop time. According to anotherembodiment, the time period may be defined by a start time, a firstduration with respect to the start time, and a second duration withrespect to the first duration. The circuit may apply the stimulatingpulse beginning at the start time and continuing for the first duration,and subsequently discontinuing the pulses during the second duration.Additional parameters may be a time period corresponding to the pulsewidth for each pulse during the series of electrical pulses, and a timeperiod corresponding to the pulse interval between each pulse. Aparameter may also include a voltage corresponding to the pulse heightfor each pulse in the series of electrical pulses.

The real time clock and the programmable calendar allow the stimulatingwaveform to vary over greater periods of time. For example, the realtime clock may supply data corresponding to a week during the timeperiod. Consequently, the waveform may be programmed to apply adifferent waveform during each particular week in the treatment period.The real time clock may also supply data corresponding to the day of theweek during the treatment period. Alternatively, the real time clock maysupply data corresponding to a month of the year during the treatmentperiod, such that the waveform may vary from month-to-month as thetreatment progresses. Moreover, the real time clock may also supply datacorresponding to the day of the month, and/or the day of the year.

Although current-controlled stimulating pulses are described above, theinvention is equally applicable to constant voltage andvoltage-controlled pulses.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified view of a preferred embodiment in accordance withthe invention.

FIG. 2 is a simplified schematic view of a component of the apparatus ofFIG. 2 in accordance with the invention.

FIG. 3 is a simplified schematic view of a component of the apparatus ofFIG. 3 in accordance with the invention.

FIG. 4 is a flow chart of steps involved in creating a table of voltagethreshold values and associated current values in accordance with theinvention.

FIG. 5 is a flow chart of steps involved in comparing sensed voltagevalues with voltage threshold values in accordance with the invention.

FIG. 6( a) is a time plot illustrating a current wave form in accordancewith the invention.

FIG. 6( b) is a time plot illustrating a prior art voltage wave formcorresponding to the time plot of FIG. 6( a).

FIG. 6( c) is a time plot illustrating a voltage wave form correspondingto the time plot of FIG. 6( a) in accordance with the invention.

FIG. 7 illustrates a data structure for storing parameters for thewaveform of a stimulating pulse in accordance with the invention.

FIG. 8 illustrates another data structure in accordance with theinvention.

FIG. 9 illustrates yet another data structure in accordance with theinvention.

FIG. 10 illustrates another data structure in accordance with theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An improved neuromuscular stimulator is illustrated in FIG. 1, anddesignated generally with reference number 10. The stimulator 10includes an implantable pulse generator 12, a lead system 14 of one ormore electrodes 16. Stimulator 10 may use voltage controlled and/orcurrent controlled stimulation in such a way to limit power drains fromthe battery and to allow accurate determination of total impedance,including lead resistance and polarization capacitance. Consequently,maintaining substantially consistent levels of power consumption maydramatically improve longevity. Stimulator 10 may limit changes in powerconsumption and may store data, which may be provided to the clinicianor used to vary stimulation parameters. For example, stored data mayinclude the occurrences of a voltage and/or current limitation during astimulation pulse. Measured parameters may be stored to correlatestimulation levels with operating conditions in order to maintainconsistent power consumption, as will be described in greater detailhereinbelow.

The implantable pulse generator 12 provides a series of electricalpulses to the stomach S. The implantable pulse generator 12 may besurgically implanted subcutaneously in the abdominal wall. Theelectrodes 16 may be installed in contact with the tissue of thestomach. Electrodes may be positioned on the outer surface of thestomach, implanted within the stomach wall, or positioned on the innersurface of the stomach wall. For example, the electrodes may be attachedto the tissue by an electrocatheter as described in U.S. Pat. No.5,423,872 to Cigaina, incorporated by reference above. Alternatively,the electrodes may be as described in copending U.S. ApplicationPCT/US98/1042 filed on May 21, 1998, and copending application Ser. No.09/122,832, filed Jul. 27, 1998, both of which are incorporated byreference in their entirety herein. As yet another alternative,electrodes may be substantially as described in U.S. patent applicationSer. No. 09/466,532 (now U.S. Pat. No. 6,606,523) and U.S. patentapplication Ser. No. 09/466,731 (now U.S. Pat. No. 6,542,776), both ofwhich were filed on Dec. 17, 1999 and are incorporated by reference intheir entirety herein.

A preferred embodiment of a current-controlled and/or voltage-controlledstimulator circuit according to the present invention is illustrated inFIG. 2, and hereinafter referred to as I/V circuit 20. I/V circuit 20may typically be housed in implantable pulse generator 12. IV circuit 20may limit power drains from the battery 22 and allow accuratedetermination of total impedance, including lead resistance andpolarization capacitance.

The I/V circuit 20 also includes a control circuit 24, a voltagemultiplier 26, storage capacitor 28, a regulated switch 32, and avoltage and/or current sensor and feedback controller 34 to regulateswitch 32. The stomach tissue stimulation occurs on leads 14, which arein turn connected to electrodes 16 (FIG. 1). Moreover, memory 25 may beprovided to store data, and display unit 27 for displaying data may beprovided.

The battery 22 may be selected in order to have long lifecharacteristics when implanted in the patient. The voltage-control andcurrent-control features of the invention may extend battery lifefurther. Preferably, the battery 22 has deliverable capacity of greaterthan 2.5 amp-hours. In a preferred embodiment, two batteries may beprovided.

The regulated switch 32 is designed to control current and/or voltagelevels either throughout the entire output stimulator pulse, e.g., byusing continuous feedback, or only at the leading edge, e.g., byselecting the appropriate initial voltage on capacitor 28 based onV_(c)=V_(control) or V_(c)=I_(control)·R_(electrode).

The shape of the current and/or voltage waveform applied to the stomachtissue is adjustable and controlled by digital means in the controlcircuit 24, which typically is or contains a microprocessor.

The timing features of control circuit 24 are illustrated in FIG. 3. Byusing a crystal 2 to control oscillator 42 (which is either internal orexternal of processor 44 which may receive input 43 from control circuit24 or provide output 45), accuracy is achieved for real-time clockcounter 46. Alternatively, the oscillator 2 and count down chain canalso be external to processor 44 and also be used to generate thestimulating waveform. Typically, a 32 or 100 kilohertz crystal clock maybe used to provide timing. Stimulation pulse width is typically 100 to500 microseconds (10 to 50 oscillations of 100 kilohertz clock), and thepulse interval may be 25 milliseconds or 2500 oscillations. The “ontime,” i.e., the period in which the pulses are applied, may be twoseconds (200,000 oscillations) for this waveform, and the “off time,”i.e., the period in which no pulses are applied, may be three seconds.It is useful to synchronize time inside the processor 44. A programmablestorage device, such as programmable calendar 48, may be used to keeptrack of different times during the treatment period, such as hours ofthe day, day of the week, etc.

With continued reference to FIG. 2, the electrodes 16 (FIG. 1) presentan impedance to the stimulating output leads 14 of I/V circuit 20. Thisimpedance may be made up of two components. The first component is aresistance due to net energy transfer from circuit 20 to the stimulatedtissue S, and the second component is a capacitance in series with theresistance due to ion transfer and charge accumulation across theelectrode-tissue interface. FIGS. 6( a)-(c) illustrate the effects ofthese components. If a pulse of constant current is sent throughelectrodes 16, as illustrated in FIG. 6( a), the resistance componentcauses a voltage to immediately appear across the electrodes (FIG. 6(b)). As the current continues to flow during the pulse, the capacitancecomponent charges up, which may contribute to a steady increase involtage during the pulse. Both of these components may vary in valuefrom patient to patient. For an individual patient, these values mayalso change after implantation due to factors such as, for example,location of electrode placement, shifts in placement, changes inphysiological conditions at the tissue interface, changes in anatomicalshape, etc.

For instance, an increase in capacitance may cause the voltage near thetrailing edge of the constant current pulse to increase without anycorresponding increase in stimulating strength. This in turn may causeincreased energy to be delivered during the pulse, and thus a higherbattery energy drain. In the case of a voltage pulse, a decrease inresistance may cause an increase in current at the leading edge of thepulse and attendant increased battery energy drain. Thus, the batteryenergy drain is limited in accordance with the invention by limiting thecurrent and/or voltage during the stimulating pulse.

In a voltage pulse, which is typically generated by charging capacitor28 to a peak value and turning on switch 32 to its maximum conductance,the initial peak current drain is only limited by switch conductance inseries with the conductance of the lead system conductors 14.

I/V circuit 20 is capable of limiting high battery energy drain due toshifts in impedance. The closure of output switch 32 is controlled bysensor/controller unit 34. In the preferred embodiment,sensor/controller unit 34 provides a novel feature of sensing both thevoltage across and the current through switch 32. Instructions on theshape and duration of the stimulating waveform are received from controlcircuit 24. The feedback controller in sensor/controller unit 34compares the actual current 341 and/or voltage 342 of switch 32 to thewave shape instructions of control circuit 24. Based on the differenceof these two signals, control circuit 24 produces an error signal tocontrol switch 32 through negative feedback. These operations can beaccomplished in either a digital or analog mode or in a combinationthereof. The switch 32 is typically an analog device, and thereforesignal 343, which is ultimately produced to control switch 32, may alsobe analog. The digital portion of this function could be accomplished incontrol circuit 24 with real-time digitized current and/or voltage data261 supplied by sensor/controller unit 34.

Another feature of switch 32 working in conjunction with control circuit24 is the ability to detect when the pulse wave form meets or exceeds acertain limit, or threshold, in voltage and/or current, to flag thatoccurrence as an “event,” and to log or store the event at the time inwhich it occurred. This event marker, along with the time during thewaveform, is available to control circuit 24 via line 261. Controlcircuit 24 may also be programmed to detect when the limit, orthreshold, is reached during a particular part of the stimulating pulse,i.e., leading edge, trailing edge, etc.

Another feature in accordance with the present invention embodied in I/Vcircuit 20 is improved accuracy in the operation of voltage multiplier26. An important novel feature is the added programmable parameter of avoltage and/or current limit or threshold value set by the clinician, inconjunction with the programmable value of either current or voltage,including the shape of the stimulating pulse (e.g., starting currentand/or voltage, ending current and/or voltage, start time, stop time,duration, etc.).

The programmable current or voltage threshold parameter is stored withother programmable information in control circuit 24. An instructionbased on this parameter is supplied to voltage multiplier 26 which iscapable of charging capacitor 28 to a large number of voltages closelyspaced in value. For example, the voltage multiplier 26 would beinstructed by control circuit 24 to charge capacitor 28 to a voltagejust slightly larger than the programmable voltage limit or threshold.The voltage multiplier 26 may also be used to control the wave shape inconjunction with sensor/controller unit 34.

The processor in control circuit 24 can adjust the voltage multiplier 26to a lower voltage to achieve the programmed limiting and thus savebattery power. The voltage multiplier 26, which may include a switchcapacitor array, may increase or decrease the battery voltage V_(B),e.g., in integer or half integer multiples thereof, such as ½ V_(B), 3/2V_(B), 2 V_(B), 3 V_(B), etc. Alternatively, voltage multiplier 26 maybe or may contain a transformer, usually in flyback mode, to changebattery voltage V_(B) in order to maintain the necessary voltage, e.g.,such that V_(m)>I_(program)*R_(electrode).

The embodiment of I/V circuit 20 allows for many programmable modes ofoperation including the modes of constant current and voltage dischargewith switch 32 turned on to maximum conductance. One of the additionalmodes includes a constant current or a controlled-current wave shapewith a separately programmable voltage limit or threshold. In this mode,the sensed current 341 may be used to regulate switch 32 throughnegative feedback. For example, the sensed voltage 342 is compared tothe programmable voltage limit. If this sensed voltage 342 reaches thisprogrammable voltage limit, the feedback may be modified to maintain thesensed voltage 342 at this limit. It is understood that the feedback maymaintain the voltage at the limit, slightly below the limit, or preventthe voltage from exceeding the limit, as deemed appropriate by oneskilled in the art. Another mode of operation is a constant voltage orcontrolled-voltage wave shape with a separately programmable currentlimit. In this mode, the sensed voltage 342 may be used to regulateswitch 32 with negative feedback. The sensed current 341 is compared tothe programmable current limit. If this sensed current 341 reaches thelimit, the feedback may be modified to maintain current at this limit.Typically, the initial voltage on capacitor 28 may be set by controlcircuit 24 to be the minimum voltage required. For instance, if theconstant current pulse is programmed to 10 milliamps and the greatesttotal impedance is 700 ohms, the initial voltage of capacitor 28 wouldbe set at a voltage slightly above seven volts such that the voltageacross capacitor 28 at the end of the pulse would be seven volts.

Certain modes of operation in accordance with the invention may notrequire the use of particular elements described in I/V circuit 20 withrespect to FIG. 2. For example, in the case of constant current or acurrent-controlled wave shape, voltage limiting may also be achievedwithout voltage sensing means in control circuit 24 by chargingcapacitor 28 to the programmed voltage limit value. A limitation involtage is achieved since the voltage across 28 is theoretically themaximum voltage that can appear across output 14. In this case, eventdetection (as described above) can be implemented by detectingsaturation or the condition of maximum conductance of switch 32. In thecase of voltage discharge with switch 32 turned on to maximumconductance, no voltage regulation is taking place and therefore thevoltage sensing of 24 is not necessary. However, current sensing isimplemented to limit current in this case.

Another feature of the preferred embodiment of IN circuit 20 is theability of a stored program or subroutine in control circuit 24 togenerate a sequence of current-controlled (and/or voltage-controlled)pulses associated with a sequence of voltage (and/or current) limitvalues, to interpret resulting limit event data, and to thereby measurethe induced voltage (and/or current) waveform shape. The IN circuit 20may store this shape for subsequent telemetry to the clinician, andanalyze the shape to calculate impedance values, including resistanceand/or capacitance components, as will be described in greater detailhereinbelow.

An additional aspect in accordance with the invention is the minimumenergy capacity of battery 22. To achieve long term therapy, e.g., for athree to five year period, the energy limiting features described aboveare preferably combined with at least a 10 watt-hr total batterycapacity. In the case of lithium type batteries the corresponding totaldeliverable current should preferably be three ampere hours.

The operation of IN circuit 20 is described with respect to FIGS. 4, 5,and 6(a)-6(c). FIG. 4 illustrates a method for setting the values of theupper limit voltage V_(max). As a result of performing the steps of FIG.4, a data table associating current values with upper voltage limits orthresholds V_(max) is compiled. The real-time clock is programmed toapply controlled-current values to the tissue at particular times duringthe treatment period. Upon implantation of the stimulator, the voltagefor each programmed current value may be measured, and a voltage limitmay be set for each current value. At step 50, an initial test voltagelimit is set, i.e., V_(lim). The current-controlled or constant currentpulse is applied at step 14. If the voltage is found to meet or exceedV_(lim) at step 54, the process proceeds to step 18. V_(max) may be setequal to V_(lim). Alternatively, V_(max) is calculated as a valuegreater than the measured V_(lim), e.g., 125% to 150% of the measuredvalue. The calculated value of V_(max) is stored, preferably in tabularform along with the associated current value, at step 18. If the voltagedoes not meet or exceed the present value of V_(lim) at step 54, thenV_(lim) is reduced by a predetermined increment at step 58, and theprocess is repeated until the voltage exceeds the value of V_(lim). Thisprocess is repeated for each current-controlled pulse value until allapplicable values of V_(max), are calculated and stored in a table.Alternatively, this process may be carried out for a voltage-controlledpulse in order to create a table of associated current limits. In such acase, an initial value of a test current limit would be set at step 50,and the voltage-controlled pulse applied at step 14. The sensed currentwould be compared with the test current limit at step 54. If the sensedcurrent is found to meet or exceed the test current limit, a currentlimit may be set at step 18. If the sensed current does not meet orexceed the test current limit, the test current limit may be lowered atstep 58, and the process of steps 14-58 repeated. This procedure forsetting a table of current/voltage limits is exemplary only, and it iscontemplated that other test procedures may be implemented.

FIG. 5, in conjunction with FIGS. 6( a)-6(c), illustrates the operationof the I/V circuit 20 in accordance with the invention during theapplication of a stimulation waveform to the tissue. The followingexemplary procedure is described with respect to a current-controlledpulse with a programmed voltage threshold, but a similar procedure wouldbe carried out for a voltage-controlled pulse with a current threshold.

At step 60, the current-controlled pulse may be applied to the stomachtissue. The sequencing of various electrical pulses is controlled by thecontrol circuit 24, described above with respect to FIG. 2. Asillustrated in a time plot in FIG. 6( a), the exemplary current pulsecommences at t1 and ends at t2. (The durations of the various signalsare not shown to scale and may have whatever duration is deemedappropriate to one skilled in the art.) Current-control switch 32maintains current at the programmed current 64.

At step 62, a value is set for an upper voltage limit, i.e., V_(max).The value of V_(max), may be fixed. Alternatively, V_(max) may beadjustable or programmable based on the circuit operating conditions,such as, for example, the magnitude of the current applied at step 60. Alookup of the tabular data compiled in FIG. 4 may be performed to set avalue of V_(max).

At step 66, the sensor/controller unit 34 measures sensed voltage anddetermines whether the voltage meets or exceeds the upper voltage limitV_(max). The sensor/controller unit 34 may be programmed to measurecontinuously or periodically store voltage data telemetry at step 66. Atime plot illustrating voltage V across the tissue is shown at FIG. 6(b). Due to the polarization capacitance effect of the stomach tissue,the voltage across the stomach tissue increases as the circuits attemptsto maintain constant current. Thus, the voltage, initially at voltage 68at t1, may increase to voltage 70 at t2. (FIGS. 6( a)-6(c) are alignedsuch that signals represented in the FIGS. in the same horizontalposition occur simultaneously.) In FIG. 6( b), the voltage meets orexceeds voltage limit V_(max) 72 at t3.

The comparison of the voltage with the voltage limit V_(max) (step 66)may occur continuously during the stimulating pulse. Alternatively, thevoltage sensor 34 may be programmed to compare the voltage with thevoltage limit V_(max) at the leading edge of the stimulating pulse,i.e., at a time period near t1. According to another alternativeembodiment, the voltage sensor 34 may be programmed to determine if thevoltage meets or exceeds V_(max) at the trailing edge of the stimulatingpulse, i.e., at a time period near t2.

If the voltage limit sensor 34 is not triggered, i.e., voltage is belowvoltage limit V_(max), operation of the circuit proceeds on path 74 ofthe flowchart of FIG. 5, and the circuit applies a current pulse to thetissue, as required by the control circuit. Voltage limit V_(max) may beset to a new value at step 62 if operating conditions require, e.g., ifthe current pulse changes.

However, if the voltage meets or exceeds voltage limit V_(max), severaloperating steps may also occur. Steps 76, 78, 80, and 82 are illustratedin sequential order. However, it is understood that steps 76, 78, 80,and 82 are independent and may occur in a different order orsimultaneously, as deemed appropriate by one skilled in the art. Certainones of these steps may also be omitted, if desired to change thefunctionality of the circuit.

At step 76, the occurrence of the “event” described above, i.e., theoccurrence of voltage meeting or exceeding the upper voltage limitV_(max), is stored, e.g., in the memory of the control circuit. In theexample of FIG. 6( b), the event occurred at time t3. The timeassociated with the event may be measured as an absolute time value,i.e., the calendar date and time, or as the elapsed time from theinitiation of the treatment, or as the elapsed time from the initiationof the particular current pulse. The occurrence of the event may beincluded in telemetry data, as with the voltage data at step 66, above.The event data point may include the current value and the voltage atthe time the event occurred, i.e., a “current value−voltage limit pair.”

With continued reference to FIG. 5, data may be calculated at step 78.For example, the total lead resistance R_(eff) may be calculated, forexample, as the ratio of V_(max) divided by the programmed current(I_(prog)). The event data point may also include associating thecurrent value with the electrode lead resistance, i.e., as an “electroderesistance pair.” This data point may also be stored in the telemetrydata. Monitoring lead resistance is useful in predicting the batterylife of the stimulator. As described above, increased resistance causesa substantial voltage drain on the battery, with associated reduction inbattery life.

The polarization capacitance may also be calculated from the data takenduring the above steps. For example, the polarization capacitance may becalculated as the ratio of the programmed current to the time rate ofchange of the voltage (i.e., C_(polarization)=I_(prog)/dV/dt). From theparameters being measured, the time rate of change of the voltage may beapproximated from the change in voltage between the voltage 68 at theleading edge t1 and the voltage 70 at the trailing edge t2 of the pulseand the time elapsed during the pulse, or the pulse width (i.e., t2−t1).Calculation of the polarization capacitance provides information onbattery drain, wherein a large capacitance may be indicative of highdrain on the battery that reduces battery life. The event data point mayalso include associating the current value with the polarizationcapacitance. This data point may also be stored in the telemetry data.

At step 80, voltage may be adjusted by using the voltage multiplier 26.For example, the control circuit 24 may be programmed to adjust thevoltage multiplier 26 to reduce voltage in integer or fractional integerincrements. With reference to FIG. 6( c), after the event occurs at t3,the voltage is adjusted to remain at the level of voltage limit V_(max)72, slightly below voltage limit V_(max) 72, or not to exceed V_(max)72.

The calculation of lead resistance and polarization capacitance at step78 may be helpful in determining the degree of voltage adjustment inorder to maintain the voltage below V_(max). A large value of electroderesistance or polarization capacitance may indicate a substantial drainon the battery. Accordingly, a substantial adjustment may be made in thevoltage at step 80. Conversely, smaller values of electrode resistanceor polarization capacitance may indicate a less substantial drain on thebattery, and a smaller adjustment to keep the voltage at or belowV_(max).

At step 82, the event data stored or calculated at steps 76-80 may begraphically displayed or listed on a display terminal or printed output.The process may continue until it is determined that the treatment iscompleted at step 84, at which time the current/voltage limitation maybe ended (step 86).

As illustrated in FIG. 3, above, I/V Circuit 20 includes a real-timeclock 46, which supplies data corresponding to the time of day duringthe treatment period, and programmable calendar 48, that can beprogrammed to store the parameters that define the above pulse train.The parameters are used by the control circuit 24 in determining thewave shape of the stimulating pulse. The parameters correspond toparticular times during the treatment. Medical observations suggest thatfood intake, digestion and other gastrointestinal functions arecircadian, that is, they operate on a 24 hour daily cycle. There arecertain periods during the day when gastric functions are less activethan other times of the day. The programmable calendar 48 can thereforeprovide increased stimulation at certain hours of the day, and decreasedstimulation at other hours of the day. Among other benefits, devicelongevity may be increased due to the energy saving of this programming.Thus the stimulators 12 may deliver stimulation pulses for a fraction ofeach hour while the patient is awake. The programmability of calendar48, described below, allows the application of longer-term circadianvariations which may likewise be beneficial to the patient and extendbattery life.

A plurality of pulse train parameters may be stored in memory associatedwith the programmable calendar 48. Sample data 90 for a treatment periodis shown in FIG. 7. The data 90 may be for a 24-hour period, such as“day one” 92, which may include calendar information 94. The pulsetrains may be stored as cycles 96. For example, pulse train parametersmay include start times 98, stop times 100, the pulsewidth 102, thepulse interval 104, the duration of the applied pulses (the “on” period)106, or the duration period in which no pulses are applied (the “off”period) 108, and the voltage of the pulse or the pulse height 109. Theprogrammable calendar 48 receives data from the clock 46 concerning thetime-of-day and the date. Programmable calendar 48 can obtain theassociated parameters from the data 90 and supply them the processor 44,accordingly. The “date” associated with the treatment may vary,depending on the expected duration of the treatment. For example, indata format 110 (FIG. 8), the data may correspond to the day of week(e.g., “day one” 112 through “day seven” 114). Each of the data pointsin day one 112 through day seven is similar to data point 90. Theprogrammable calendar 48 may function on a seven-day cycle whereinprogrammable calendar accesses day one after day seven in a continuousloop 116. Thus, each day of the week could have a particular sequence ofstimulating pulse train parameters. As a result, the pulse train isprogrammed to stimulate the stomach tissue in the same way on the sameday of each week.

As illustrated in FIG. 9, the data format 120 may refer to a simple,numbered day in a periodic sequence of days, such as the numbered daysof the year (i.e., “day one” 122 through day 365” 124), or the numbereddays within a month (e.g., “day one” 112 through “day 31”, not shown).The calendar 48 would then cycle back to the first data point asindicated by arrow 126. As illustrated in FIG. 10, the data format 130may be hierarchical and thus may recognize intermediate time periods,such as weeks 132 and/or months (not shown) within a treatment period.For example, it may recognize that the treatment is at “week two” 134 or“week three” 136, in addition to the elapsed number of days. Thecalendar 48 could be programmed to so that the pulse generator 10 isturned off for a number of weeks. The generator may then be turned onone day a week, During the next week, the generator may be turned on fortwo days a week, etc. Each sequence of cycles (see, FIG. 7) within agiven “on” day, could also be different from the previous “on” day.

The programmability of the pulse train wave forms based on the dateprovides the ability to turn the above stimulating pulse train on or offor increase or decrease waveform parameters over increasingly longermeshed periods of time.

An alternative embodiment of the neuromuscular stimulator describedabove includes an additional mode for stimulating the neuromusculartissue of the gastrointestinal tract. The neuromuscular stimulatorapplies a series of primary electrical pulses to the tissue as describedhereinabove. These electrical pulses may be applied during a first timeinterval, and may be discontinued during a second time interval. Asecondary series of pulses having a lower voltage may be applied to thetissue during the second time interval, i.e., when the primaryelectrical pulses are discontinued. The resulting current flowingbetween the stimulating electrode pair is measured.

Data, including, for example, sensed current, may be measured. Thecurrent data may be analyzed for changes over time. From this analysis,statistics may be computed. For example, a statistic which may becomputed is the time period in which the changes in the current datarepeats. This time period may be used to approximate the peristalticaction of the tissue. It may be desirable to change the rate ofperistaltic activity, i.e., to slow down or increase the rate thereof,by varying the series of electrical pulses based on the statistics asdescribed above.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention.

1. A neuromuscular stimulator for stimulating tissue of thegastrointestinal tract with a series of electrical pulses applied to theneuromuscular tissue during a treatment period, comprising: a real timeclock configured to supply data corresponding to the time of day duringthe treatment period; a programmable calendar configured to store aparameter for defining said series of electrical pulses, the parameterhaving a reference to a respective time of day during the treatmentperiod; a control circuit configured to apply said series of electricalpulses defined by the parameter at the respective time of day.
 2. Theneuromuscular stimulator defined in claim 1, wherein the parameter is atime period during which said series of electrical pulses are applied tothe neuromuscular tissue.
 3. The neuromuscular stimulator defined inclaim 2, wherein the time period comprises a start time with referenceto the time of day and a duration with respect to the start time, andwherein the control circuit is configured to apply the series ofelectrical pulses beginning at the start time and extending for theduration.
 4. The neuromuscular stimulator defined in claim 2, whereinthe time period comprises a start time with reference to the time ofday, a first duration with respect to the start time, and a secondduration with respect to the first duration, and wherein the controlcircuit is configured to apply the series of electrical pulses beginningat the start time and continuing for the first duration, anddiscontinuing the series of electrical pulses during the secondduration.
 5. The neuromuscular stimulator defined in claim 2, whereinthe time period comprises a first time with reference to the time of dayand a second time with reference to the time of day, and wherein thecontrol circuit is configured to apply the series of electrical pulsesbeginning at the first time and discontinuing the series of electricalpulses at the second time.
 6. The neuromuscular stimulator defined inclaim 1, wherein the parameter is a time period corresponding to thepulse width for each pulse in the series of electrical pulses.
 7. Theneuromuscular stimulator defined in claim 1, wherein the parameter is atime period corresponding to the pulse interval between each pulse inthe series of electrical pulses.
 8. The neuromuscular stimulator definedin claim 1, wherein the parameter is a voltage corresponding to thepulse height for each pulse in the series of electrical pulses.
 9. Theneuromuscular stimulator defined in claim 1, wherein the real time clockis configured to supply data corresponding to a week during saidtreatment period.
 10. The neuromuscular stimulator defined in claim 1,wherein the real time clock is configured to supply data correspondingto a day of the week during said treatment period.
 11. The neuromuscularstimulator defined in claim 1, wherein the real time clock is configuredto supply data corresponding to the month of the year during saidtreatment period.
 12. The neuromuscular stimulator defined in claim 11,wherein the real time clock is configured to supply data correspondingto a day of the month during said treatment period.
 13. Theneuromuscular stimulator defined in claim 1, wherein the real time clockis configured to supply data corresponding to a day of the year duringsaid treatment period.
 14. A method of stimulating neuromuscular tissueof the gastrointestinal tract with a series of electrical pulses appliedto the neuromuscular tissue during a treatment period, comprising:supplying data corresponding to the time of day during the treatmentperiod; storing a parameter for defining said series of electricalpulses, the parameter having a reference to a respective time of dayduring the treatment period; and applying said series of electricalpulses defined by the parameter at the respective time of day.
 15. Themethod defined in claim 14, wherein supplying the time of day comprises:providing a real time clock.
 16. The method defined in claim 14, whereinstoring the parameter further comprises: providing a programmablecalendar for storing the parameter.
 17. The method defined in claim 14,wherein storing the parameter further comprises: storing the parameteron the programmable calendar.
 18. The method defined in claim 14,wherein the parameter is a time period, and wherein the storing theparameter further comprises: storing the time period during which saidseries of electrical pulses are applied to the neuromuscular tissue. 19.The method defined in claim 18, wherein the storing the time periodcomprises: storing a start time with reference to the time of day, andstoring a duration with respect to the start time; and applying theseries of electrical pulses beginning at the start time and extendingfor the duration.
 20. The method defined in claim 18, wherein thestoring the time period comprises: storing a start time with referenceto the time of day, storing a first duration with respect to the starttime, and storing a second duration with respect to the first duration;and applying the series of electrical pulses beginning at the start timeand continuing for the first duration, and discontinuing the series ofelectrical pulses during the second duration.
 21. The method defined inclaim 18, wherein the storing the time period comprises: storing a firsttime with reference to the time of day, and storing a second time withreference to the time of day; and applying the series of electricalpulses beginning at the first time and discontinuing the series ofelectrical pulses at the second time.
 22. The method defined in claim18, wherein the parameter is a time period, and wherein the storing theparameter further comprises: storing the time period corresponding tothe pulse width for each pulse in the series of electrical pulses. 23.The method defined in claim 18, wherein the parameter is a time period,and wherein the storing the parameter further comprises: storing thetime period corresponding to the pulse interval between each pulse inthe series of electrical pulses.
 24. The method defined in claim 18,wherein the parameter is a voltage, and wherein the storing theparameter further comprises: storing the voltage corresponding to thepulse height for each pulse in the series of electrical pulses.
 25. Themethod defined in claim 14, wherein the supplying data corresponding tothe time of day further comprises: supplying data corresponding to aweek during said treatment period.
 26. The method defined in claim 25,wherein the supplying data corresponding to the time of day furthercomprises: supplying data corresponding to a day of the week during saidtreatment period.
 27. The method defined in claim 14, wherein thesupplying data corresponding to the time of day further comprises:supplying data corresponding to the month of the year during saidtreatment period.
 28. The method defined in claim 27, wherein thesupplying data corresponding to the month of the year further comprises:supplying data corresponding to the day of the month during saidtreatment period.
 29. The method defined in claim 14, wherein thesupplying data corresponding to the time of day further comprises:supplying data corresponding to the day of the year during saidtreatment period.